In the United States, lung cancer kills more people than breast, colon and pancreatic cancers combined1. The past decade of research has brought to the forefront a need to accurately subdivide lung cancers based on a combination of pathologic features, gene expression, response to therapy, and oncogene/tumor suppressor mutation status in order to personalize treatment. Several groups have identified a neuroendocrine subtype of non-small cell lung cancer (NSCLC) that has a much poorer prognosis than typical NSCLC2-3. Gene expression signatures from large cohorts of lung tumors have suggested that distinct neuroendocrine cancers appear in 5-8% of NSCLC2. Despite the discovery of this NSCLC subtype in patient tumors, a complete molecular characterization has been lacking because a similar subtype has yet to be identified in a significant number of lung cancer cell lines. Preliminary data that consists of gene expression signatures, response to therapy, and oncogene mutation status in a large cohort of NSCLC cell lines can be used to identify a panel of lung cancer cell lines that fit a neuroendocrine phenotype. To determine the clinical and therapeutic significance of the neuroendocrine-NSCLC (NE- NSCLC) subtype, it is important to first study this subtype in vitro and in vivo by identifying a panel of cell lines that display neuroendocrine characteristics. My initial data and literature suggests that the gene expression signature of neuroendocrine-NSCLC cell lines clusters together with that of small cell lung cancer lines, which are driven by powerful neuroendocrine-specific proteins4, particularly the ASCL1 transcription factor. The primary goals of this research are to 1) elucidate the role of ASCL1 and 2) identify novel biomarkers and targets for therapy in a molecularly and clinically distinct subtype of NSCLC. Phenotypic response to knockdown of ASCL1 in NSCLC cell lines will determine this gene's functional relevance, and high-throughput chromatin immunoprecipitation-sequencing (ChIP-Seq) will determine genes that are transcriptionally regulated by ASCL1. The goals will be accomplished by rigorous molecular and biochemical analysis of ASCL1 in NSCLC cell lines and mouse xenograft models of lung cancer, and by comprehensive ChIP-Seq and functional siRNA screening methods to discover pathways controlled by ASCL1. These data can then be used to postulate mechanisms of ASCL1-induced tumorigenesis and exploited for potential therapeutic application by selecting NE-NSCLC patient tumors with my newly derived biomarker and then giving them therapy directed at ASCL1-relevant targets.